Crucible for melting silicon of highest purity and method of making it



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United States Patent 01 3,051,555 Patented Aug. 28, 1962 ice 3,051,555 CRUCIBLE FOR MELTING SILICON OF HIGHEST PURITY AND METHOD OF MAKING IT Theodor Riimme], Munich, Germany, assignor to Siemens and Halske Aktiengesellschaft Berlin and Munich, a corporation of Germany Filed Mar. 17, 1958, Ser. No. 721,895 Claims priority, application Germany Apr. 15, 1957 3 Claims. (Cl. 23-2235) This invention is concerned with a crucible for melting silicon of highest purity and a method of making it.

Extreme purity of the melt is a basic requirement in the melting of silicon, especially for the production of semiconductor crystals. The use of crucibles introduces the danger of contaminations entering into the melt from the material of the crucible. Socalled crucible-free methods have, therefore, been applied which circumvent this danger. However, these methods call for considerable technical expenditure and it has, accordingly, been the endeavor to construct crucibles which exclude the danger of contamination from the crucible material. Thus, for example, crucibles for melting silicon of highest purity have been tried, made of SiO;, BeO, A1 or made of similar high-melting material indifferent to silicon, such as carbide of titanium or zirconium and the like, or lined on the inside with these materials. However, completely satisfactory results could not be achieved, because these materials always cause contamination of the melt however slight such contamination might be.

The object of the invention is to provide a'crucible suitable for melting silicon of highest purity which completely excludes the danger of contamination resulting from the crucible material. This crucible may always be used when a method is employed for the heating of the melt in which the required heat is not transmitted to the melt by conduction of the crucible wall but in the melt itself, by radiation or by high frequency energy. The crucible according to the invention is primarily intended for the melting of silicon of highest purity; however, it may with sensible modification also be used for melting other highly pure semiconductor material.

It is in accordance with the present invention proposed to use for the melting of highly pure silicon or other semiconductor material, preferably for electrical purposes, for

' example, for rectifiers or transistors, a crucible made of thermally well conductive material with a melting point lying under that of the melt. The cruciblebody is to'be lined on the inside thereof with a layer or coating of purest silicon (or other semiconductor material to be melted), so thick, and the crucible body is to be exteriorly cooled by means of a cooling agent to such an extent, that the temperature in the material of the crucible and the coating will be under the melting temperature of the coating material and of the material of the crucible body.

The crucible body is made of a material of good thermal conductivity, for example, a metal such as copper. The cooling agent acting through the wall of the crucible body, therefore, can effectively protect the silicon coating on the interior surface against the action of the molten material. It is most favorable if the crucible body is surrounded with a jacket preferably of the same material, spaced therefrom, with a stationary or circulating cooling agent, for example, water, contained in the intermediate space formed therebetween.

The various objects and features of the invention will become apparent from the description which will be rendered below with reference to the accompanying drawing. In the drawing,

FIG. 1 shows an embodiment of a crucible according to the invention;

FIG. 2 represents a crucible body comprising an induction coil sealed with insulating material of silicon disposed between the individual windings thereof; and- FIGS. 3a and 3b show two views of another embodiment.

Referring now to FIG. 1, representing an embodiment of a crucible for melting silicon according to the invention, numeral 11 indicates the copper crucible body provided with a copper jacket connected therewith, numeral 12 indicating the coating or lining of highly pure silicon, 13 indicating the melt in the crucible and 14 indicating a cooling agent. The circulation of the cooling agent or liquid may be controlled, if desired, by means such as a wall 15. The material 13 is heated in known manner, for example, by an electrical gas discharge or by a heat radiator or an electron beam, in a vacuum or in a suitable gas atmosphere, for example, argon, hydrogen, helium, and brought to melting condition. High frequency energy is preferably used for the heating of the material. It is in such case, however, necessary to preheat the material to produce therein a sutiicient initial conductivity and to arrange the coil 16, delivering the high frequency electromagnetic energy, in such a manner that the flux delivered thereby can enter into the material without hindrance. The induction coil 16 is for these reasons disposed directly above the surface of the material to be melted.

It is important in this connection that the metallic crucible body and silicon coating or lining 12 on the inside thereof remain so cool that the silicon lining is not melted by the molten material 13 and that it does not alloy with the metal of the crucible body. It is, therefore, advisable to supervise the temperature of the crucible during the operation. This may be effected by a thermo couple which may be disposed in a bore of the crucible body. The melting point of the silicon is far higher than that of the copper; care must be taken to avoid heating the crucible to the melting point of the copper of which it is made.

If the heating of the material is to be effected by high frequency, the crucible body may, in accordance with a feature of the invention, be combined with the required conduction coil. A crucible for melting silicon of highest purity and meeting this requirement is shown in FIG. 2. The corresponding crucible comprises an induction coil having windings 21 wound to form the crucible body. The individual turns or windings may be sealed by means of an insulating material 22, preferably silicon, disposed therebetween. The crucible body thus produced is coated or lined on the inside with a layer 23 of silicon of highest purity, at least throughout portions thereof which will be contacted by the melt 24. The induction coil is suitably Wound from a copper tubing through which is circulated a cooling medium. Material to be melted is introduced into the crucible and the coil is in known manner connected with a suitable high frequency source adapted to deliver sufiicient energy. The dimensions of the copper tubing and of the coil body formed thereby, and the thickness of the silicon coating 23 are correlated with or matched to the temperature of the cooling medium circulating through the tubing, so that the cooling compensates the Joul heat developed inside the crucible body while the fusing of the melt to the silicon layer 23 or melting thereof to the copper tubing is avoided.

It is in the above described embodiment not absolutely necessary that the turns or windings of the coil body are completely sealed elf by the insulating material 22. In case of closely arranged windings, the insulating material 22 may even be omitted. The surface tension of the melt jointly with the effect of the ponderomotoric forces produced by the field, will prevent flowing off of the melt. If it is, however, desired to seal the windings of the coil body with silicon, such can be done by briefly submerging it in molten silicon. The resulting crucible body after cooling,

3 must be provided with the coating or lining 23 of silicon of highest purity.

A variant of a crucible according to the invention may also be obtained by using a socalled energy concentrator, for example, in the form as shown in FlGS. 3a and 3!). FIG. 3b is a section along the plane of slot 33 and FIG. 3a is a section perpendicular to such plane.

The energy concentrator comprises a cylindrical cop per body 31 provided with means forming a concentric basin 32 for receiving the material to be processed. It is important to provide the crucible body 31 which is made of metal, that is, of a good conductive material, with a longitudinal slot 33 extending preferably to the axis A, so as to prevent formation of an annular shunt for the igh frequency energy, thereby also effecting concentration of the field of the high frequency coil 35 to the inside of the crucible, that is, to the material 36 which is to be processed and melted.

This concentrator forms together with the field coil 35,

preferably concentrically surrounding it, and with the material 36 contained in the basin 32 and to be processed, a transformer with close coupling between the field coil 35 and the material 36. The basin 32 for holding the material 36 is coated with a liner 34 of silicon of highest purity. The body 31 is hollow and adapted to receive a cooling medium which may or may not circulate therethrough. It is not absolutely necessary that the slot 33 and the hole 33 at the bottom of the basin which serves the same purpose as the slot, be sealed, for example, with silicon.

The present invention is also concerned with a method of providing on the interior surfaces of the crucible body to be used for melting silicon of highest purity, a coating or liner of purest silicon. The described crucible body is for this purpose introduced into an atmosphere of pure hydrogen and pure silicon halide, such as SiHCl SiCl and heated (to about 900 C.) so as to cause precipitation or deposition of purest silicon, from the gaseous phase, upon the crucible body and particularly upon its inside surfaces thereof. It is thereby particularly suitable to heat the crucible body by the action of high frequency electromagnetic alternating fields. The gas may be held at normal atmospheric pressure of 1 atm. The gas mixture is conducted so as to fiow past the hot crucible body, so as to remove the formed halide-hydrogen from the reaction zone. The silicon separation may, if desired, be effected or supported by an electrical discharge. The pressure of the gas atmosphere is in such a case adjusted so as to ob tain the desired type of discharge.

The method of the invention may also be used in making crucibles for the melting of other semiconductor substances, for example, germanium, boron, selenium, etc. There are in such cases likewise gaseous or volatilizable combinations which decompose in the presence of heat or an electrical discharge, and, if desired, in the presence of a reducing agent such as hydrogen, yielding pure semiconductor material, that may thereupon be processed in a manner analogous to the processing of silicon halides, for the production of semiconductor lining upon the crucible body.

Changes may be made within the scope and spirit of the appended claims.

I claim:

1. A method of producing silicon crystals of highest purity, wherein the semiconductor material is melted in a crucible and the crystal obtained by drawing from the melt, comprising the steps of placing the semiconductor material to be melted into a crucible formed of a material which is electrically and thermally of good conductivity, constructed for operation as anelement of an electrical high frequency heating system, lining said crucible on its inside with a highly pure coating, deposited from the gas phase, of the semiconductor material that is to be melted, melting the material to be processed by the action of highfrequency current flowing through the crucible body, the latter being slotted so that the field of the high frequency current is directed into the melt, cooling the crucible during the melting of the processing material to a temperature below the melting temperature of the crucible body and below the temperature of the semiconductor melt, and controlling the cooling to compensate for the foul heat developed inside the crucible body and thereby maintain the semiconductor lining of the crucible in a solidified state and prevent fusing of the melt to the semiconductor lining.

2. A method of producing silicon crystals of highest purity as defined in claim 1, comprising the steps of introducing the crucible body in which such melt is to be formed in a gaseous atmosphere of hydrogen and pure halide of the semiconductor material to be melted, effecting a separation of pure semiconductor material therefrom and deposit of such material upon the inside of said crucible body, withdrawing the crucible from said atmosphere, and introducing the semiconductor material to be processed to form-such melt into said crucible, preparatory to the heatingthereof.

3. A method of producing silicon crystals of highest purity as defined in claim 1, comprising the steps of introducing the crucible body formed from a metal of good electrical and thermal conductivity into a gaseous atmosphere of purest hydrogen and pure halide of the semiconductor material to be melted, subjecting said body in said gaseous atmosphere to the action of an electrical gas discharge deposit from said gaseous atmosphere to deposit purest semiconductor material upon the inside of the crucible body, and introducing the semiconductormaterial to be melted into the crucible preparatory to the heating thereof.

References Cited in the file of this patent UNITED STATES PATENTS 589,221 Pacet Aug. 31, 1897 2,354,876 Owens Aug. 1, 1944 2,793,103 Emeis May 21, 1957 2,817,509 Solomon Dec. 24, 1957 2,818,248 Kelsey Dec. 31, 1957 2,836,412 Krieger May 27, 1958 2,858,586 Brennan Nov. 4, 1958 2,872,299 Celmer Feb. 3, 1959 2,941,867 Maurer June 21, 1960 FOREIGN PATENTS 14,020 Great Britain June 15, 1912 OTHER REFERENCES Germany $38055 VI/40d dt. 14 June 1956 

1. A METHOD OF PRODUCTING SILCICON CRYSTALS OF HIGHEST PURITY, WHEREIN THE SEMICONDUCTOR MATERIAL IS MELTED IN A CRUCIBLE AND THE CRYSTAL OBTAINED BY DRAWING FROM THE MELT, CONPRISING THE STEPS OF PLACING THE SEMICONDUCTOR MATERIAL TO BE MELTED INTO A CRUCIBLE FORMED OF A MATERIAL WHICH IS ELECTRICALLY AND THERMALLY OF GOOD CONDUCTIVITY, CONSTRUCTED FOR OPERATION AS AN ELEMENT OF AN ELECTRICAL HIGH FREQUENCY HEATING SYSTEM, LINING SAID CRUCIBLE ON ITS INSIDE WITH HIGHLY PURE COATING, DESPOSITED FROM THE GAS PHASE, OF THE SEMICONDUCTOR MATERIAL THAT IS TO BE MELTED, MELTING THE MATERIAL TO BE PROCESSED BY THE ACTION OF HIGHFREQUENCY CURRENT FLOWING THROUGH THE CRUCIBLE BODY, THE LATTER BEING SLOTTED SO THAT THE FIELD OF THE HIGH FREQUENCY CURRENT IS DIRECTED INTO THE MELT, COOLING THE CRUCIBLE DURING THE MELTING OF THE PROCESSING MATERIAL TO A TEMPERATURE BELOW THE MELTING TEMPERATURE OF THE CRUCIBLE BODY AND BELOW THE TEMPERATURE OF THE SEMICONDUCTOR MELT, AND CONTROLLING THE COOLING TO COMPENSATE FOR THE JOUL HEAT DEVELOPED INSIDE THE CRUCIBLE BODY AND THEREBY MAINTAIN THE SEMICONDUCTOR LINING OF THE CRUCIBLE IN A SOLIDIFIED STATE AND PREVENT FUSSING OF THE MELT TO THE SEMICONDUCTOR LINING. 